The invention described herein was made in the performance of work under NASA Grant Number NAG3-446 and in subject with the provisions of 35 U.S.C. 202.
The present application relates to new silicon based preceramic polymers useful in the production of silicon carbide reinforcement fibers, and to the novel synthesis procedures by which such polymers may be produced.
It is well known that silicon carbide can provide highly desirable and unique properties due to its chemical inertness, high temperature stability, semi-conductor properties, and especially its extreme hardness. Fibers of silicon carbide can be used in a wide variety of applications, particularly as reinforcements for composites.
The problems encountered in prior attempts to utilize silicon carbide reinforcements in most applications were due to the great difficulty experienced in attempting to form the silicon carbide reinforcement material into the desired configuration. One suggested means of overcoming these problems was to employ silicon based preceramic polymers, such as polycarbosilanes. These polycarbosilanes are prepared by a controlled pyrolysis of polydimethylsilanes, which in turn are typically prepared from dimethyldichlorosilane and Na metal as shown below. ##STR2## wherein R is hydrogen or methyl
From the hexane soluble non-volatile polymer fraction, fibers could be melt spun, which could be subsequently cross-linked by surface oxidation in air, and then pyrolized at high temperature in an inert atmosphere, to ultimately yield silicon carbide fibers. Such procedures are, for example, suggested by Yajima et al., synthesis of continuous silicon carbide fibers with high tensile strength and high Young's modulus, Journal of Materials Science, 13, 1978 (2569-2575), who postulated a structure as follows: ##STR3## The structure has many cyclized units in it and it is well known that poly(dimethylsilanes) tend to cyclize and volatilize when heated.
It was subsequently suggested that volitilization could be prevented by replacing some of the methyl groups by phenyl groups; however the yield of silicon carbide in fact dropped, since the ultimate percent of volatilization increased. These procedures also tended to result in the formation of significant yields of graphite. In general, the methods heretofore employed produced a rather low "char yield", that is to say a rather low ultimate yield of silicon carbide as a function of the preceramic polymer (after it had been formed into the desired shape, but prior to the oxidative cross linking and/or high temperature pyrolization steps.)
It will of course be obvious that such weight losses are highly undesirable, not only from the economic point of view but also from the point of view of the resultant inherent problems of shrinkage, displacement and the like. For this reason it has long been desired to discover a silicon based preceramic polymer having a much higher yield of pyrolized silicon carbide fiber, preferably at least in the range of from about 50 to 75%.